1. Field of the Invention
The present invention relates to six-degrees-of-freedom parallel mechanisms and, more particularly, to a six-degrees-of-freedom parallel mechanism for micro-positioning work, the mechanism consisting of three external links coupling a moving platform to a base platform and three internal links commonly coupled to one multiaxial spherical joint, with the external and internal links individually having a linear actuator structure capable of precisely controlling the position of the link.
2. Description of the Prior Art
In accordance with the recent trend of increased demand and increased importance of production and machining of high precision parts, production of semiconductors, microsurgery, gene manipulation and cell conformity in a variety of industrial fields, such as a high precision engineering field, a semiconductor manufacturing field, a medical field and a genetic engineering field, the study and development of robots or manipulators for micro-positioning work has been actively carried out.
In the prior art, a variety of serial robots with open links have been used as such manipulators for micro-positioning work in a variety of industrial fields. Due to their open links, such serial robots are somewhat advantageous in that they preferably provide a large workspace, and preferably accomplish improved manipulability. However, these serial robots are problematic in that they inevitably create accumulated errors at their end effectors since they have serial actuators. The serial robots are thus undesirably deteriorated in their operational accuracies. Another problem experienced in the conventional serial robots resides in that their operational performance is undesirably reduced, particularly when they are used in high-speed work or other work with excessively variable weight of dynamic load.
In an effort to overcome such problems experienced in the conventional serial robots, a variety of parallel mechanisms have been actively studied since the 1980s. Such parallel mechanisms have a closed chain structure, and so they are free from actuator-caused errors accumulated at their end effectors in addition to preferably having a high structural strength different from the conventional serial robots, even though the workspace provided by the parallel mechanisms is regrettably smaller than that of the serial robots. The parallel mechanisms thus accomplish a desirably high operational performance when they are used in high-speed work or other work with excessively variable weight of dynamic load. Therefore, it is more preferable to use such parallel mechanisms in place of the conventional serial mechanisms for micro-positioning work.
Such parallel mechanisms are structurally advantageous in a variety of items as described above, and so they can be preferably used in micro-positioning work. However, in order to allow such parallel mechanisms to effectively carry out a variety of desired works within a limited workspace, it is necessary that such a parallel mechanism have six degrees of freedom. That is, in a parallel mechanism for micro-positioning work, it is necessary to set the degree of positional precision of actuators to 20 xcexcm or less, and the resolution of position sensors to 5 xcexcm or less in addition to providing effective workspace of at least 20 mm.
FIG. 1 is a perspective view of a conventional three-degrees-of-freedom micromotion in-parallel actuated mechanism. FIGS. 2a and 2b are perspective views of conventional micro hand modules using six-degrees-of-freedom parallel link mechanisms. For ease of description, the two terms xe2x80x9cthree-degrees-of-freedomxe2x80x9d and xe2x80x9csix-degrees-of-freedomxe2x80x9d will be referred to simply as xe2x80x9c3dofxe2x80x9d and xe2x80x9c6dofxe2x80x9d herein below.
As shown in FIG. 1, Lee analyzed and experimentally established the kinematics and dynamics of a 3dof micromotion in-parallel actuated manipulator using a piezo-actuator (Lee, K. M., 1991, xe2x80x9cA 3dof Micromotion In-Parallel Actuated Manipulatorxe2x80x9d, IEEE Transactions on Robotics and Automation, Vol. 7, No. 3, pp. 634xcx9c641).
As shown in FIGS. 2a and 2b, Arai and Stoughton fabricated and analyzed super-precise 6dof parallel micro hand modules using a piezo-actuator (Arai, T. and Stoughton R., 1992, xe2x80x9cMicro Hand Module using Parallel Link mechanismxe2x80x9d, ASME, Proceedings of the Japan U.S.A, Book No. 10338A, pp. 163xcx9c169).
On the other hand, in an effort to point out and solve the problems and limitations of conventional serial robots, Hollis proposed a 6dof micromotion parallel mechanism using a magnetically levitated actuator (Hollis, R. L., 1991, xe2x80x9cA 6dof Magnetically Levitated Variable Compliance Fine-Motion Wrist: Design, Modeling, and Controlxe2x80x9d, IEEE Transactions on Robotics and Automation, Vol. 7, No. 3, pp. 320xcx9c332).
The object of the mechanisms proposed in the above-mentioned three cited references is to perform micro-positioning work with a high resolution using a piezo-actuator or a magnetically levitated actuator having a high degree of positional precision, on a level of micrometers. However, in said mechanisms, the linear displacement of the piezo-actuators is limited to 10 xcexcm or less, the linear displacement of the magnetically levitated actuator is limited to about 5 xcexcm, and the workspace is limited to about 4 xcexcm. Therefore, the above mechanisms cannot be effectively or properly used as general 6dof mechanisms for micro-positioning work since such general 6dof mechanisms require a degree of positional precision of actuators set to 20 xcexcm or less, a resolution of position sensors set to 5 xcexcm or less in addition to an effective workspace of at least 20 mm as described above.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a 6dof parallel mechanism for micro-positioning work, which freely performs a desired motion of translation and rotation of a moving platform using internal and external links actuated by linear actuators capable of precisely controlling the link positions, thus being effectively used as a 6dof parallel mechanism required to carry out a variety of micro-positioning works within a limited workspace.
In order to accomplish the above object, the present invention provides a 6dof parallel mechanism for micro-positioning work, comprising: a moving platform placed at an upper position, with a multiaxial spherical joint mounted to the central portion of the moving platform; a base platform placed at a lower position; a plurality of external links coupling the moving platform to the base platform; and a plurality of internal links coupling the multiaxial spherical joint of the moving platform to the base platform.
In the 6dof parallel mechanism of this invention, the internal links are commonly coupled to the multiaxial spherical joint so as to form a tetrahedral structure.
In addition, each of the internal and external links is a linear actuator designed to be precisely controllable in its position.
The 6dof parallel mechanism of this invention also preferably enlarges the workspace to at least 20 mm, and preferably reduces the number of ball and socket joints of the moving platform from six to three, thus enlarging the area of rotation of the moving platform. In addition, the desired motion of rotation of the moving platform of this mechanism is performed by use of three linearly actuated external links.